Construction Of CoMn₂O₄ Spinels And Their Performance For Catalytic Oxidation Of NH₃

With the increase in human activities?such as the burning of fossil fuels,the production of chemical fertilizers,and the use of decorative materials,etc.?,NH₃ as one of the the toxic and harmful gases produced in human activities,plays an increasingly important role in the generation of smog effect.The selective catalytic oxidation?SCO?with low energy consumption and no secondary pollution is an ideal and promising treatment process for controlling NH₃ emissions,and the core of the technology is the design and construction of the catalyst.In recent years,more and more attention has been paid to the development of composite oxide catalysts.How to effectively control the structure of the composite oxide catalyst to achieve both high NH₃ catalytic activity and N₂ selectivity has become the focus of research.The composite oxide with spinel structure has high stability,and there is a strong interaction between the internal metal elements,which can form a good redox couple,resulting in enhancing the redox ability of the catalyst.Therefore,Co and Mn were used as the A-site and B-site metals of the spinel in this paper,and the CoMn₂O₄ spinel catalyst was prepared by the precipitation method.Furthermore,the CoMn₂O₄-WO₃ catalyst was synthesized by acid modification on the CoMn₂O₄ spinel surface.The relationship between spinel structure and NH₃catalytic activity was explored.The reaction mechanism of spinel in NH₃-SCO reaction and the effect of surface structure on the selective catalytic oxidation of NH₃ were discussed.Detailed research contents include:1.Ammonia was used as both precipitant and coordination agent,and a CoMn₂O₄ spinel catalyst?denoted as CMO-T?with a large specific surface area and abundant defects was ynthesized via a two-step precipitation and low-temperature?200^oC?annealing process.It was revealed that CMO-T sample showed the Co-enriched surface structure which was caused by the abundant Co³⁺species partially diffused into the spinel octahedral sites.The presence of Co³⁺species not only helped to enhance the redox performance of the spinel,but also had a strong chemical adsorption performance for NH₃.And abundant defects of CMO-T samples could shorten the internal metal-oxygen bonds,resulting in high mobility of oxygen species in the spinel.CMO-T catalysts could completely convert NH₃ at 200^oC,and the N₂ selectivity was60%.2.The CMO-T sample was modified via soaking in tetrabutylammonium hydroxide?TBAOH?solution and impregnating with doped WO₃,and the CoMn₂O₄-WO₃ catalyst was obtained?denoted as CMO-SD-W?.The reults showed that TBAOH could change the surface structure of the CoMn₂O₄ spinel without destroying the spinel crystal structure,which could increase the amount of reactive sites,and adjust the interaction between WO₃ and CoMn₂O₄spinel.Surface acid modification promoted the adsorption and activation of NH₃ on CMO-SD-W.CMO-SD-W catalysts could completely convert NH₃ at 180^oC,the N₂ selectivity increased to 85%,and they worked stably for more than 36 hours.